Branched tetrasiloxanes as typified by methyltris(trimethylsiloxy)silane are used as industrial oil, cosmetic oil and detergents. For the tetrasiloxane used as cosmetic oil among other applications, high purity is a critical requirement. Specifically, it is desired to reduce the content of impurity ingredients of one silicon atom below 0.1% by weight because they are highly irritant to the skin. It is also desired to minimize the content of linear and cyclic compounds of two or three silicon atoms because they are also highly irritant to the skin.
It is thus desirable that the methyltris(trimethylsiloxy)silane used as cosmetic oil be highly pure with a minimized content of low-boiling silicon-containing ingredients.
Several methods for the preparation of methyltris(trimethylsiloxy)silane are known in the art.
(1) Co-hydrolysis of methyltrichlorosilane and trimethylchlorosilane in the presence of methanol. See International Patent Publication 2001/15658 and JP-A 2002-68930.
(2) Reaction of methyltrichlorosilane with hexamethyldisiloxane in the presence of a perchloric acid catalyst. See Dokl. Akad. Nauk., SSSR, 1976, 227, 362-365.
(3) Reaction of methyltriethoxysilane with hexamethyldisiloxane in the presence of an acidic ion-exchange resin. See J. Organomet. Chem., 1988, 340, 31-36.
(4) Reaction of methyltrialkoxysilane with hexamethyldisiloxane in the presence of a carboxylic acid and an acid catalyst. See JP-A 11-217389.
(5) Addition of conc. sulfuric acid to methyltrimethoxysilane, hexamethyldisiloxane and methanol, followed by dropwise addition of a mixture of water and methanol. See International Patent Publication 2001/15658 and JP-A 2002-68930.
These methods, however, have drawbacks. Method (1) needs a large amount of water and have very low yields due to a low selectivity of reaction. Method (2) is inadequate from an industrial aspect since the catalyst used is perchloric acid which is awkward to handle. Method (3) must use a large excess of hexamethyldisiloxane in order to increase the rate of reaction. Nevertheless, a large amount of the reaction intermediate, 1,1,1,3,5,5,5-heptamethyl-3-ethoxytrisiloxane is left behind. The method is thus unsuitable for obtaining methyltris(trimethylsiloxy)silane in high purity form. In Method (4), there are produced not only the desired methyltris(trimethylsiloxy)silane, but also a noticeable amount of a by-product having a similar boiling point, 1,1,1,3,5,5,5-heptamethyl-3-methoxytrisiloxane. Although the yield at the reaction solution stage is 82%, it is estimated that the yield upon isolation becomes substantially lower where distillation is effected to obtain highly pure methyltris(trimethylsiloxy)silane. Method (5) produces highly pure methyltris(trimethylsiloxy)silane, but is unsatisfactory because of a yield as low as 70%.
None of prior art methods are capable of preparing branched tetrasiloxanes in good yields, especially methyltris(trimethylsiloxy)silane of a sufficiently high purity to use as cosmetic oil in good yields.